The invention is in the field of paste intaglio printing inks for line-engraved plates, The inks of the invention have improved dispersibility and chemical resistance.
Line-engraved intaglio printing is typically used for printing security documents, such as banknotes, and uses printing cylinders having engravings therein in which intaglio printing inks have been deposited. Paste inks used in such printing are substantially different in nature from inks used in other forms of printing such as gravure, offset and ink-jet printing.
In line-engraved intaglio printing, the paste ink is applied to the printing cylinder so as to fill the engravings and then the excess ink on the cylinder (i.e., the ink which is outside of the engravings) is typically removed by a wiping cylinder which wipes off the excess ink. A diluted aqueous solution of sodium hydroxide, i.e. an alkaline (caustic) solution, is normally used by the wiping cylinder to emulsify and remove the excess ink. Therefore, in order to achieve effective wiping of the printing cylinders, it is important that the ink readily disperses in the wiping solution.
The poor dispersibility of known commercial paste inks in the alkaline washing solutions used in the cylinderwipe intaglio printing processes is often a problem and, to deal with this problem, attempts are often made to improve dispersibility by adding surfactants to the ink. However, the addition of surfactants usually results in a rapid loss of resistance to solvents and chemicals, particularly at higher surfactant concentrations, such as 2-10 weight %.
Disadvantageously, when using a caustic wiping solution, it is necessary to have ultrafiltration equipment to recover caustic values from the washing solution containing the suspended inks and extra chemicals, such as sodium or potassium hydroxide. Furthermore, sulfuric acid is needed to neutralize the used caustic solutions. By contrast, simple tap water would be ideal for use with the wiping cylinder because of the environmental advantages due to reduction in use of chemicals. However, the paste intaglio printing inks, which are generally used in the industry, do not disperse sufficiently in municipal water.
To print on a substrate using the line-engraved intaglio printing process, after the engravings have been inked and the excess ink removed, the printing cylinder is pressed against the substrate using a very high compression of about 10,000 N per centimetre of the cylinder""s length. Once the printed matter is dry (typically after several days of oxidative curing) it is desirable that it remains steadfast on the substrate and resistant to leakage or smudging on coming into contact with solvents or chemicals such as alkaline solutions.
Accordingly, it is highly desirable that the properties of a paste intaglio printing ink include both good dispersibility and good chemical/solvent resistance and, in particular, that such dispersibility include dispersibility in water.
It is known in the art of making of inks and coatings that dispersibility can be improved by using alkyds with high acid numbers, but such components do not provide the desired dual properties of chemical resistance and water dispersibility.
In U.S. Pat. No. 4,764,215 it is claimed by Rudolph that drying oil soaps, prepared by high temperature saponification of unsaturated oils using metal hydroxides in the presence of triethanolamine, give inks with improved wipe-ability and resistance to water and eliminate volatile organic solvents However, as disclosed in U.S. Pat. No. 34,389 to Amon et, al., the ink compositions claimed by Rudolph are not satisfactory because they do not allow stable incorporation of even very low amounts of water, they do not sufficiently dry after printing and they are not resistant to the action of alkaline solutions.
It is also known in the art that the problems associated with an ink""s lack of chemical resistance can be partly solved by the addition of components known to have high chemical resistance, such as dehydrated castor oil or epoxy esters of unsaturated fatty acids, but such additives usually result in significantly reduced dispersibility of the modified ink. Also, some such additives have poor solubility and require the use of a compatibilizing solvent, such as glycol ether. The addition of such low molecular weight compatibilizing materials (i.e. solvents) causes an increase in the total concentration of volatile organic solvents which introduces a formulation problem because some jurisdictions, for environmental reasons, restrict the allowable concentration of such solvents in printing inks.
Therefore, there is a serious need in the printing industry for a paste intaglio printing ink having improved solvent and chemical resistance as well as improved dispersibility, including tap water dispersibility, while, at the same time, avoiding the use of high concentrations of volatile organic components.
In accordance with the invention there is provided a paste intaglio printing ink comprising: an alkyd having a degree of neutralization within the range of about 10% to about 35%, preferably about 15% to 30%, an acid number in the range of about 30 to about 110, preferably about 60 to about 90, and being in the amount of about 15 to about 50 weight percentage; an anionic or nonionic surfactant in the amount of 0.5-12 weight percentage; and an organic multifunctional base in the amount of 0.5-5 weight percentage and selected from the group consisting of multifunctional alcohol-amines, or substituted polyethyl-eneimines, wherein the substituted polyethyleneimines are modified with epichlorohydrine or alkoxylated and the base has a boiling point higher than 150xc2x0 C. The intaglio ink normally also comprises pigment in the amount of 0-15 weight percentage, an extender in the amount of 25-70 weight percentage and a wax compound in the amount of 0-15 weight percentage. Optionally, the ink may include an unsaturated oily component in the amount of 5-15 weight percentage.
It is preferable that the surfactant has low molecular weight and that the molecular structure of the surfactant comprise bulky hydrophobic moieties.
The intaglio ink may also comprise water in an amount of up to about 15 weight percentage.
Also, in accordance with a further embodiment of the invention, there is provided a paste intaglio printing ink comprising a zwitterionic surfactant in the amount of 0.5-12 weight percentage and not separately including an organic base.